All references cited and discussed below in this specification are incorporated herein by reference in their entirety and to the same extent as if each reference was individually incorporated by reference. In the case of conflicting terminology, the present disclosure shall control.
Growing concerns about the environment have resulted in the development of new environmentally friendly technologies, new materials, and new ways to reduce and minimize wastes. One of the wastes produced by contemporary society in abundant quantity is municipal sewage sludge, euphemistically often referred to as biosolids. Biosolids are a mixture of exhausted biomass generated in the aerobic and anaerobic digestion of the organic constituents of municipal sewage along with inorganic materials such as sand and metal oxides.
Various methods have been proposed for the disposal of municipal sewage sludge, including ocean dumping, landfilling, cropland application and incineration. Other methods that have been used to dispose of or utilize municipal sewage sludge include road surfacing, conversion to fertilizer, compression into building blocks, and carbonization.
The application of raw sewage sludge as a fertilizer produces odor problems and is also associated with the risk of contamination of the soil by heavy metals and pathogens. A more efficaceous and safer alternative is the pyrolytic carbonization of sludge to obtain useful sorbents.
Carbonization of sludge in the presence of chemical activating agents, such as zinc chloride and sulfuric acid produces sorbents, having applications in processes such as removal of organics and chlorinated organics in the final stages of water cleaning.
The process of carbonization of biosolids has been studied in detail using different chemical agents and various conditions. In general, materials obtained as a result of the treatment have surface areas between 100 and 500 m2/g, but their performance as adsorbents has been demonstrated to be much worse than that of activated carbons.
Since hydrogen sulfide (H2S) is the main source of odor from waste water treatment plants, the possibility of using sewage sludge as a source of adsorbents for H2S is appealing. Because sewage sludge contains a considerable amount of organic nitrogen, carbonization of such species can lead to the creation of basic nitrogen groups within the carbon matrix which have been proven to be important in the oxidation of H2S. Another advantage to the use of sludge as a starting material is the presence of significant amounts of iron that is added to the raw sludge as a dewatering conditioner. This is considered to be a catalyst for H2S oxidation.
Primarily, caustic-impregnated carbons have been used as adsorbents of hydrogen sulfide at waste water treatment plants. Because of the presence of caustic compounds, such as KOH or NaOH, their pH is high, and hydrogen sulfide is oxidized to elemental sulfur. The process is fast and caustic-impregnated carbons have high hydrogen sulfide breakthrough capacity, around 140 mg/g, measured using accelerated test.
Caustic-impregnated carbons, although efficient for H2S removal, have many disadvantages. The disadvantages of caustic-impregnated carbons include the following:    1) limited capacity for physical adsorption of volatile organic compounds (VOCs) due to the presence of caustic materials in the carbon pore system;    2) low self-ignition temperature which may result in fire inside the carbon vessel;    3) special safety precautions in dealing with caustic materials have to be applied;    4) high density because of the presence of water; and    5) higher cost than that of unmodified carbons.
Thus, there is a great interest in the development of new types of adsorbents for use in waste water treatment facilities.
The following patents and publications provide relevant background to the present invention:
U.S. Pat. Nos. 3,619,420, 3,998,756, 3,887,461, 4,122,036, and 5,356,849; Adib et al., J. Colloid Interface Sci., 1999, 214, 407–415; Cariaso et al., Carbon, 1975, 13: 233–239; Chiang et al., Can. J. Chem. Eng., 1987, 65: 922–927; Katoh et al., Appl. Cat. B: Environ., 1995, 6: 255–262; Lu et al., Fuel, 1995, 74: 344–348; Lu et al., Gas Sep Purif., 1996, 10: 103–111; Lu et al. Environ Tech., 1995, 16, 495; Piskorz et al., Ind. Proc. Des. Dev., 1996, 25: 265–270; and Stejns et al., Ind. Eng. Chem. Prod. Res. Dev., 1977; 16: 35–41.